Trap for preventing diffusion pump backstreaming

ABSTRACT

An oil-backstreaming trap is located in a vacuum line between an oil diffusion pump and the evacuated system so that gases from the system are drawn through the trap on the way to the pump. First and second baffle plates extend partially across the crosssectional area of the trap and are spaced apart from each other along the direction of gas flow. The surface of the baffle plate closest to the evacuated system is smooth. The other baffle plate surfaces are roughened to form a large plurality of raised and depressed portions such as are formed by milling the roughened surfaces to form a large plurality of pyramid-shaped elements. In this manner, oil vapor from the diffusion pump which tends to backstream into the evacuated system, is coalesced on the roughened surfaces to form oil drops which then fall back into the diffusion pump.

United States Patent [72] Inventor Harold Shapiro 1.557.442 10/1925Eichelman 55/464 Silver Spring, Md. 1.572.245 2/1926 Quinn 55/446 [21]Appl. No. 789,045 1,807,983 6/1931 Hegan et al 55/444 [22] Filed Dec.3l,1968 2.204.489 6/1940 Gray 209/506 [45] Pa en ed N v-30.19 2.386.29910/1945 Downing... 230/101 [73] Assignee The United States of Americans2.478.934 8/1949 Morse 2 r pr n y th A mini rat r f h 3.048.039 8/1962Hackler 55/186 National Aeronautics and Space 3.175.373 3/1965Holkelboer et a1. 62/555 Administration 3.231.490 1/1966 Fry 261/1 12Primary E.\un1iner-Frank W. Lutter 54 TRAP FOR PREVENTING DIFFUSION PUMPAssismn! E.\-aminerBernard NOZiCk BACKSTREAMING Al!me vs--R. F. Kempf.E. Levy and G T. McCoy 10 Claims, 1 1 Drawing Figs.

[52] U.S.Cl 417/152, ABSTRACT: An oil-backstreaming trap is located in11 5/446. 55/464 vacuum line between an oil diffusion pump and theevacuated [51] lnt.Cl F04f 9/06 system so that gases from the system aredrawn through the [501 Field of Search ..55/46l465. tra on th way to thepump. First and second baffle plates ex- 439,442446; /1 tend partiallyacross the cross-sectional area of the trap and 209/506;261/112;417/55,158. 152 are spaced apart from each other along the direction of gasflow. The surface of the baffle plate closest to the evacuated [56]Rem'emes cued system is smooth. The other baffle plate surfaces areUNITED STATES PATENTS roughened to form a large plurality of raised anddepressed 433,992 /1892 Cochrane /464 portions such as are formed ymilling the roughened surfaces 1192 450 1919 Fisher 55 4 4 to form alarge plurality of pyramid-shaped elements. In this 3354 214 7 19 9417/154 manner. oil vapor from the diffusion pump which tends to 23 33 09 Nuorivaam |61l139 backstream into the evacuated system, is coalescedon the 3 33335 19 5 Butler 2 555 roughened surfaces to form 011 dropswhich then fall back into 3,310,227 3/1967 Milleron 62/555 the diffusionP p- 520,675 5/1894 Gindele 55/444 EVACUATED SYSTEM TRAP E PUPmimmuuvsmsn 3,623,828

FIG 6 INVENTOR HAROLD SHAPIRO @As BY ATTORNEYS TRAP FOR PREVENTINGDIFFUSION PUMP BACKSTREAMING The invention described herein was made byan employee of the United States Government and may be manufactured orused by or for the Government for governmental purposes without thepayment of any royalties.

BACKGROUND OF THE INVENTION Oil diffusion pumps are frequently used toevacuate highvacuum systems. In such cases, oil from the pump has atendency to migrate and/or backstream into the evacuated system so as toboth contaminate the system and prevent it from reaching the desiredvacuum. Hence, oil-backstreaming traps are often located between thepump and the system to prevent such migration.

One common type of oil-backstreaming trap employs a container for acryogen such as liquid nitrogen. Such containers are arranged so as topresent a cold surface to the flow of gas between the vacuum chamber andthe pump. In this manner, the gas molecules that are drawn out of thesystem merely bounce off of the cold surface, but the heavy oilmolecules freeze and become trapped" on the cold surface. These traps,however, are quite expensive to both construct and maintain. In thisrespect, such traps only maintain their desired low temperatures fortypical periods of about 8 to hours or so. It is an object of thisinvention, therefore, to provide a trap that is less expensive tomanufacture than the cryogen-type; and does not require an additionalexpense to maintain the traps cryogen.

Another type of oil-backstreaming trap employs various types of filtersfor preventing the diffusion pumps oil molecules from the backstreaminginto the evacuated system. Some such traps provide a tortuous paththrough which the oil molecules must travel; others merely provide a bedof particulate materials such as molecular sieves, or charcoal. Thesetypes of traps, however, have two major disadvantages. Firstly, theyhave a tendency to become saturated with oil at which time oilbackstreaming increases. Secondly, they obstruct the flow path of gasmolecules from the evacuated system so as to cut down on the diffusionpumps conductance and thereby reduce its pumping speed and efi'iciency.Consequently, it is another object of this invention to provide a trapwhich does not saturate; and which provides far less obstruction to thepumps conductance so that the pump maintains a desirably high pumpingspeed.

SUMMARY OF THE INVENTION In the past, particularly in connection withthe cryogen types of traps, it has been through of that thebackstreaming oil migrated or crept in films along the trap and filtersurfaces. For this reason, the surfaces of the cryogen traps have beenmade smooth to accommodate many layers of the frozen oil. Similarly, thefilters have been designed so as to have the largest possible amount ofsurface area to accommodate the largest possible film area prior to thefilters becoming saturated. This invention, on the other hand, is basedupon an observation that the backstreaming oil from the diffusion pumptravels mostly in the form of molecules or droplets, rather than films.In this respect, in accordance with a principle of this invention, theoil backstreaming is prevented by coalescing the oil molecules ordroplets on a roughened surface extending partially across the trapscross section and made up of high and low portions so as to form dropsof oil at the low portions whereupon, when large enough, the drops fallback toward the diffusion pump.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects,features, and advantages of the invention will be apparent from the moreparticular description of preferred embodiments thereof as illustratedin the accompanying drawings. The drawings are not necessarily intendedto be to scale, but rather are presented so as to illustrate theprinciples of the invention in clear form.

In the Drawings FIG. I is a schematic illustration of anoil-backstreaming trap located between an oil difi'usion pump and asystem evacuated by the pump;

FIG. 2 is a cross section of a preferred embodiment of the trap of theinvention taken along the line 2-2 of FIG. 1;

FIG. 3 is a cross-sectional view of a preferred embodiment of the trapof the invention taken along the lines 3-3 in FIG. 1;

FIG. 4 is a cross section of a trap of the invention taken along thelines 4-4 in FIG. 2;

FIG. 5 is an enlarged fragmentary view of a roughened baffle plate takenalong the arc 5-5 in FIG. 3;

FIG. 6 is a sectional view taken along the lines 6-6 of FIG.

FIG. 7 is a perspective view of a portion of the pyramided surfaceillustrated in FIG. 5;

FIG. 8 is an enlarged fragmentary view of an alternative type ofroughened surface corresponding to that illustrated in FIG. 5;

FIG. 9 is an enlarged fragmentary view of still another type ofroughened surface;

FIG. 10 is an enlarged fragmentary view of yet another alternative typeof roughened surface; and,

FIG. 11 is a cross-sectional view of FIG. 10's roughened surface taltenalong the lines I I-ll thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I an oil-backstreamingtrap I2 is located between an evacuated system I3 and an oil diffusionpump I4. Such pumps are used where vacuums of greater than about 0.01torr are desired. They operate by heating a liquid such as heavy oil ormercury to form a curtain or vapor thereof for entrapping molecules ofunwanted gas from the evacuated system. In this respect, it should benoted that although the instant invention is described in terms of atrap for preventing oil backstreaming, it is equally useful to preventother types of molecules from backstreaming into evacuated systems.

As is illustrated in more detail in FIGS. 2, 3, and 4, the trap iscomprised of four walls 16, I8, and 20, and 22 joined together to form asquare, open-ended box adapted to be fastened on one end to the vacuumsystem system and on the other end to the diffusion pump. Morespecifically, the trap is to be inserted between the pump and the vacuumchamber in any suitable manner such as fastened, put in a drawer, orheld in a loose configuration, for example. A first baffle plate 24extends across the traps cross section from the rear wall 18 to thefront wall 22 and declines downwardly at about 5 percent declinationfrom the wall 16 to an edge 26 thereof so as to cover about percent ofthe traps cross sectional area. The surface 28 of the baffle plate 24adjacent the evacuated system 13 is smooth so that gas molecules drawnfrom the evacuated system will not adhere thereto. The underside 30 ofthe plate 24, however, is roughened in a manner to be described in moredetail shortly. The trap may be formed of aluminum, stainless steel orcopper, for example.

A second baffle plate 32 also extends across the traps cross sectionfrom wall 18 to wall 22; and declines downwardly on about a 3 percentslope from sidewall 20 to edge 34 thereof so as to extend across aboutpercent of the trap's cross-sectional area. Both of the baffle plate 325major surfaces 36 and 38 are roughened.

In operation, gas molecules M from the evacuated system 13 pass throughthe traps entry throat 40 located between the upper plate 24's edge 26and the sidewall 20. They then bounce back and forth between theroughened surfaces 30 and 36 of the upper and lower baffle plates,respectively, (as illustrated by the solid line of a typical moleculespath in FIG. 4;) and exit from the trap's exit throat 42 located betweenthe lower plate 38's edge 34 and the sidewall 16. Oil molecules from thediffusion pump 14, however, are heavier and become trapped in thedepressions of surfaces 30, 36, and 38. The thrusly trapped moleculesthan coalesce to form drops of oil (or other diffusion pump liquid)which fall or drip off of the roughened surfaces back toward thediffusion pump 14.

In one embodiment of the invention, the roughened surfaces were obtainedby repeatedly passing a milling head over the plate's surface to form aplurality of small pyramids 44 as illustrated in FIGS. 5-7. In thisrespect, the pyramids were equilateral so as to have 60 faces as shownin FIG. 6; and a depth d of about 25 percent of the plate's thickness;or about 0.06 of an inch in one embodiment having a plate thickness ofabout 0.25 inch.

ln another embodiment, the roughened surfaces were comprised of a largeplurality of conical elements 46 such as are illustrated in FIG. 8.Still another embodiment, the surfaces are roughened in the form of awafflelike pattern comprised of a plurality of elements 48 (FIG. 9); andin yet another embodimerit, the surfaces are merely roughened by meansof bristle pads 50 and 52 having bristles 54 extending outwardlytherefrom.

A series of operational tests were conducted on an embodiment of theinvention having the surfaces 30, 36 and 38 roughened in accordance withthe pyramid structures illustrated and described in connection withFIGS. 57. This test structure was placed between an evacuated system andan oil difi'usion pump; and subjected to actual operation at a vacuum ofless than torr for a period of mo hours. In this respect, a quartzcrystal microbalance was used to detect any oil that backstreamed intothe evacuated system through the trap of the invention. There was nodetectable backstreaming, however, during the entire l00-hour testperiod.

A standard cryogen-cooled, chevron-baffle type of trap was then insertedbetween the pump and the evacuated system and it too, was subjected to100 hours of operation at a vacuum of less than 10 torr. The quartzcrystal microbalance, however, indicated that the standard,chevron-baffle structure permitted l.5 micro-g./cm."/minute tobackstream from the diffusion pump into the evacuated system.

The above-noted test on a preferred embodiment of the invention ratherdramatically illustrates the improved results that are obtainable by useof the invention s structure. Moreover, the structure of the inventiondoes not require the addition of a cryogen at periodic intervals duringthe traps use; nor does it reduce the pumps conductance as is the casewith the filter type of traps.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it

will be understood by those skilled in the art that various changes inform and details may be made therein without departing from the spiritand scope of the invention. For example, although the inventions traphas been described in connection with uncooled baffle plates, it will beappreciated that the principles of the invention would be equally asapplicable if the baffle plates are cooled so as to perhaps freeze andthereby trap water molecules from the evacuated system to prevent themfrom being drawn into the diffusion pump. In addition, while the traphas been illustrated as a square openended box, it will be appreciatedthat it may be alternatively round, circular, oval or, more generally,any configuration that fits the space available. Finally, it will beappreciated that while the invention provides a trap that does not use acryogen fluid, it can be used with acryogen-type trap if desired. or itcan be used with an appropriate heating means for bake out.

What is claimed is:

l.-Apparatus for preventing backstreaming oil molecules from a diffusionpump from entering a system being evacuated said apparatus comprised of:

a conduit located between and connecting said pump and said system beingevacuated thereby;

a first impervious baffle plate extending from one side of said conduittoward an opposite side partially across and over the major portion ofthe cross-sectional area of said conduit;

a second impervious baffle plate extending partially across and over thema or portion of said cross-sectional area of said conduit from saidopposite side toward side on side,

said second bafi'le baffle plate being spaced apart from said firstbaffle plate in the direction from said evacuated a system to saiddiffusion purrip; each of said baffle plates having a first surfacethereof facing said evacuated system and a second surface thereof facingsaid diffusion pump;

said second surfaces being roughened surfaces formed of an arrangementof closely spaced projections defining alternate raised and depressedportions fonning a plurality of closely spaced small pockets fortrapping the oil molecule and coalescing said backstreaming oilmolecules from said diffusion pump to fonn drops thereof in said smallpockets.

2. The apparatus of claim 1 wherein said roughened surface is comprisedof a large plurality of pyramid-shaped elements.

3. The apparatus of claim 1 wherein said roughened surface is comprisedof a large plurality of conical shaped elements.

4. The apparatus of claim 1 wherein said roughened surface comprises awaffle pattern. v

5. The apparatus of claim I wherein said first baffle plate declinesfrom said one side of said conduit toward said diffusion pump at a slopeof about 5 percent and extends across about 70 percent of thecross-sectional area of said conduit.

6. The apparatus of claim 1 wherein the surface of said said secondbaffle plate are roughened; said second bafile plate declines from saidanother side of said conduit downwardly toward said diffusion pump at aslope of about 3 percent; and

extends across about 75 percent of cross-sectional area of said conduit.I

7. The apparatus of claim 5 wherein said baffle plate declines from saidone side of said conduit toward said diffusion pump at a slope of about5 percent and covers about 70 percent of the cross-sectional area ofsaid conduit.

8. The apparatus of claim 7 wherein said roughened surfaces arecomprised of a large plurality of pyramid-shaped elements.

9. The apparatus of claim 7 wherein said roughened surfaces arecomprised of a largeplurality of conical shaped elements.

10. The apparatus of claim 7 wherein said roughened surfaces comprise awaffle pattern.

1. Apparatus for preventing backstreaming oil molecules from a diffusion pump from entering a system being evacuated said apparatus comprised of: a conduit located between and connecting said pump and said system being evacuated thereby; a first impervious baffle plate extending from one side of said conduit toward an opposite side partially across and over the major portion of the cross-sectional area of said conduit; a second impervious baffle plate extending partially across and over the major portion of said cross-sectional area of said conduit from said opposite side toward said one side, said second baffle plate being spaced apart from said first baffle plate in the direction from said evacuated system to said diffusion pump; each of said baffle plates having a first surface thereof facing said evacuated system and a second surface thereof facing said diffusion pump; said second surfaces being roughened surfaces formed of an arrangement of closely spaced projections defining alternate raised and depressed portions forming a plurality of closely spaced small pockets for trapping the oil molecules and coalescing said backstreaming oil molecules from said diffusion pump to form drops thereof in said small pockets.
 2. The apparatus of claim 1 wherein said roughened surface is comprised of a large plurality of pyramid-shaped elements.
 3. The apparatus of claim 1 wherein said roughened surface is comprised of a large plurality of conical shaped elements.
 4. The apparatus of claim 1 wherein said roughened surface comprises a waffle pattern.
 5. The apparatus of claim 1 wherein said first baffle plate declines from said one side of said conduit toward said diffusion pump at a slope of about 5 percent and extends across about 70 percent of the cross-sectional area of said conduit.
 6. The apparatus of claim 1 wherein the surfaces of said second baffle plate are roughened; said second baffle plate declines from said another side of said conduit downwardly toward said diffusion pump at a slope of about 3 percent; and extends across about 75 percent of cross-sectional area of said conduit.
 7. The apparatus of claim 6 wherein said first baffle plate declines from said one side of said conduit toward said diffusion pump at a slope of about 5 percent and covers about 70 percent of the cross-sectional area of said conduit.
 8. The apparatus of claim 7 wherein said roughened surfaces are comprised of a large plurality of pyramid-shaped elements.
 9. The apparatus of claim 7 wherein said roughened surfaces are comprised of a large plurality of conical shaped elements.
 10. The apparatus of claim 7 wherein said roughened surfaces comprise a waffle pattern. 